The present invention relates to an improvement in photosensitive imaging systems of the type taught in U.S. Pat. Nos. 4,399,209 and 4,440,846, wherein a hygroscopic polymer is incorporated in the internal phase of a microencapulated photosensitive material or in a coating composition containing photosensitive microcapsules. It is believed that the hygroscopic polymer prevents the microcapsules from drying out upon storage.
Photohardenable imaging systems employing microencapsulated radiation sensitive compositions are the subject of commonly assigned U.S. Pat. Nos. 4,399,209; 4,416,966; 4,440,846 and 4,766,050. These imaging systems are characterized in that an imaging sheet including a layer of microcapsules containing a photohardenable composition in the internal phase is image-wise exposed to actinic radiation. In the most typical embodiments, the photohardenable composition is a photopolymerizable composition including a polyethylenically unsaturated compound and a photoinitiator and is encapsulated with a color former. Exposure to actinic radiation hardens the internal phase of the microcapsules. Following exposure, the imaging sheet is subjected to a uniform rupturing force by passing the sheet through the nip between a pair of pressure rollers. An image transfer system in which the developer material is coated on a separate substrate as a separate developer or copy sheet is disclosed in commonly assigned U.S. Pat. No. 4,399,209. A self-contained imaging sheet in which the encapsulated color former and the developer material are co-deposited on one surface of a single substrate as one layer or as two interactive layers is disclosed in commonly assigned U.S. Pat. No. 4,440,846. An imaging system comprising a support, a layer containing microcapsules, a layer of developer material, and a layer containing an opacifying agent is disclosed in commonly assigned U.S. Pat. No. 4,766,050. The opacifying agent can form a separate layer or can be part of the layer containing the microcapsules or both but is interposed between the microcapsules and the developer to hide the microcapsules when viewing the image.
When imaging systems, particularly the self-contained imaging systems of the type described in U.S. Pat. No. 4,440,846 and recently allowed copending U.S. application Ser. No. 08/570,658 filed Dec. 11, 1995, now U.S. Pat. No. 5,783,353 are exposed and developed after having been stored for a period of time, the images have a tendency to darken over time. This darkening has been attributed to the presence of undeveloped color precursor outside the capsule which gradually reacts with the developer material and causes the image to darken.
Accordingly, it is desirable to provide an imaging system wherein this undesirable tendency to darken over time is reduced or eliminated.